An accurate theoretical model based on thermoelasticity theory and Fresnel diffraction integral is developed to describe the photothermal deflection (PTD) signal with a continuous-wave modulated Gaussian beam excitation. A PTD experiment is performed to investigate the dependence of PTD amplitude on the experimental parameters, such as the radius, waist position, and wavelength of the probe beam, and the detection distance. Good agreement between the experimental and theoretical results is obtained. The results reveal that the optimal detection distance highly depends on the probe beam waist position and wavelength, and the PTD amplitude can be enhanced by optimizing the probe beam radius and waist position as well as the detection distance. Moreover, it is demonstrated experimentally that the PTD amplitude is inversely proportional to the probe beam wavelength by using three probe lasers with a wavelength of 375 nm, 543 nm, and 632.8 nm. Therefore, the measurement sensitivity of PTD technique could be enhanced by using a short-wavelength probe beam.